UPDATE 3-Exxon shuts UK pipeline as police investigate fuel theft

LONDON, April 21 (Reuters) - Exxon Mobil shut one of Britain's main underground fuel pipelines, it said on Monday, after police found a stash of diesel they believe was siphoned off by thieves.

Police arrested two men, aged 32 and 34, in connection with the theft. Media reported that more than 30,000 litres of diesel, worth 41,000 pounds ($70,000) at pump prices, was involved.

Police launched their investigation on Thursday after a large quantity of diesel was found in a industrial storage unit at West Wellow, Hampshire, southern England, where the thieves are thought to have tapped into the 14-inch pipeline.

The Midline Pipeline runs north from Esso's Fawley oil refinery near Southampton to its terminal in Birmingham, central England.

Two men from the Salisbury area were arrested late Sunday and were being held in custody for questioning on Tuesday, police said.

"Lines of enquiry include establishing the exact amount of fuel stolen and examining the suspected sophisticated method used in this alleged theft," police said in a statement.

"The fuel is stored safely and securely and is being retrieved by engineers from the nearby Esso refinery."

Exxon, known as Esso in Britain, said in a statement: "Esso is committed to the very highest standards of safety. The pipeline has therefore been closed down, and our specialists are working to check it and, if necessary, effect any repair."

Stealing oil from pipelines is relatively rare in Britain, but the theft of metals such as copper cable has led to communications and rail network disruptions. In Nigeria, theft from pipelines is estimated to cost the government up to $1 billion per month.

Esso is making alternative supply arrangements and does not expect its retail customers to be affected by the pipeline closure, a company spokesman said.

The pipeline has a spur to Birmingham Airport, although this is rarely used now as most fuel supplied to the airport goes by road, he said. (Additional reporting by Belinda Goldsmith; Editing by Robin Pomeroy)